Changing The Future

The Potters have a set of twins, a boy, the son of James, and a daughter that's not James's but Poseidon's.

Poseidon possessed James when they were conceived. the story takes place in the original harry potter time line. Harry Potter and Percy Jackson are the same person.

After Voldemort attacks the Potters, their son Jamie is hailed as the boy who lived when actually it's Poseidon's daughter, Andromeda, who goes by Andi. The Potters neglected her for the next year when Poseidon steps in and takes her.
he takes her to a Witch named Sally Jackson.

Shortly after the goddess Artemis was made an Olympian, a prophecy was given that foretold of a Demigod daughter of Poseidon, and that they would be soulmates. Artemis still makes her vow to remain a virgin, but with the loop whole that she would be able to break the vow when she met her soulmate.

Four thousand years later, Artemis had almost lost hope of ever meeting her. When the big three swore on the Styx to never have kids again, she loses all hope. That is until about 35 years later when Andromeda was born.

The Lightning thief and Sea of monsters pretty much stay the same with some minor tweaks.

it's when we get to the Titans curse that the big changes happen in the timeline.

I don't know how to incorporate any of the Harry Potter books just yet, only because Andi never went to Hogwarts.

All rights belong to Rick Riordan and J.K. Rowling. I do not own either series